The present invention relates to a method of reducing interference between two communication systems operating in adjacent frequency bands, and more particularly, to a method of reducing interference between two communication systems operating in adjacent frequency bands and coexisting in a device with a limited space.
With the progress of wireless network technology and the growing demand for mobile communication, communication devices as laptops or cell phones supporting various wireless communication standards, e.g. Bluetooth, WiFi, 4G (WiMAX, LTE, and etc. . . . ) or 3G, is expected to be a must in modern life for a more convenient access to the wireless network.
WiMAX and WiFi systems operates on adjacent frequency bands, which are the 2.4 GHz Industrial Scientific and Medical (ISM) band and the licensed 2.3 GHz and 2.5 GHz bands. Most WiMAX systems use Time Division Duplex (TDD) to allocate radio resources because TDD has higher spectral efficiency and can be implemented with lower complexity. A TDD frame is divided into a downlink subframe and an uplink subframe. A WiFi system is a contention-based system without dedicated radio resource and uses a technique called carrier sense multiple access with collision avoidance (CSMA/CA). With CSMA/CA, a WiFi station monitors the wireless medium and decides when to transmit packets. When the wireless medium is detected to be idle, the WiFi station is permitted to transmit; and when the wireless medium is detected to be busy, the WiFi station defers its transmission for a random period following the wireless medium turns to be idle again.
Please refer to FIG. 1, which is a diagram of a WiMAX network 10 according to the prior art. A WiMAX-WiFi coexistence device 100 is a combination of a WiMAX system and a WiFi system, which can be a WiMAX-WiFi portable router or a WiMAX-WiFi customer-premises equipment (CPE) and performs as a WiMAX router connecting with a WiMAX base station 102 and a WiFi access point (AP) connecting with WiFi stations 104 as laptops or multimedia players. Therefore, the WiFi stations 104 are able to access the WiMAX network and communicate with WiMAX subscriber stations 106 via the WiMAX-WiFi coexistence device 100.
Since the guard band between the 2.4 GHz band for WiFi and the 2.3/2.5 GHz band for WiMAX are narrow and the portable WiMAX-WiFi coexistence device has limited dimensions, mutual interference between WiMAX and WiFi systems in the portable WiMAX-WiFi coexistence device is hard to be reduced.
In the WiMAX-WiFi conexistence device, when the WiFi system (which performs as a WiFi AP) is transmitting packets during a WiMAX downlink subframe, the WiFi transmitter interferes with the WiMAX receiver and decreases efficiency of WiMAX downlink reception, which has to be improved because the WiMAX system uses licensed bands and the waste of bandwidth is not acceptable. On the other side, when the WiFi system is receiving packets during a WiMAX uplink subframe, the WiMAX transmitter interferes with the WiFi receiver and WiFi packets cannot be received successfully, which causes a problem that the WiFi station may use the lower MCS (Modulation and Coding Scheme) rate according to its rate adaptation mechanism, and finally, interference is getting more serious until the WiFi stations and the WiFi system are disconnected.
There are several ways to reduce the mutual interference in the portable WiMAX-WiFi coexistence device. One is using RF filters in the RF front-end of the WiMAX and WiFi systems, which does help but is not cost-effective. Another one is using an arbitration procedure, such as the packet traffic arbitration (PTA) procedure as defined for WiFi-Bluetooth coexistence, to coordinate sharing of radio resources dynamically based on traffic load or traffic priority of the two systems.
However, when the WiMAX-WiFi coexistence device uses conventional arbitration procedure to try to reduce interference, the WiMAX-WiFi coexistence device does not take WiFi station connecting with WiMAX-WiFi coexistence device into consideration. The WiFi station cannot get information about the interference inside the WiMAX-WiFi coexistence device, and may still send packets to the WiMAX-WiFi coexistence device and wait for acknowledgements. On the other side, the WiFi system of the WiMAX-WiFi coexistence device may not receive packets successfully due to the mutual interference and does not send acknowledgement as response. As a result, the WiFi station will use the lower MCS (Modulation and Coding Scheme) transmission rate according to its rate adaptation mechanism, and the connection quality and channel utilization between the WiMAX-WiFi coexistence device and the WiFi station is getting worse due to the retransmissions and inefficiency MCS.